In a communications network, it is often desirable to identify and distinguish one transmitter from other transmitters operating within the network. For example, in the radio telephone industry, a cellular telephone system utilizes an electronic serial number (ESN) and a mobile telephone identification number (MIN) to provide a unique identification for each transmitter. When an individual subscriber or other authorized user of a particular cellular telephone wishes to place a phone call, he dials in a telephone number and presses the "Send" button. In response, the cellular telephone transmits its ESN and MIN to the cellular network so the individual subscriber can be charged for the telephone call.
Unfortunately, unscrupulous individuals illegally operate cellular telephones by counterfeiting the ESN and MIN of a valid subscriber's telephone in order to obtain illegal access to the cellular network without paying for the service. The ESN and MIN of a cellular telephone can be obtained by a counterfeiter electronically monitoring the initial transmission of the telephone, and then programming the detected ESN and MIN into another telephone for illegal use. Thus, the mere transmission of the authentic ESN and MIN is by itself inadequate to protect a cellular telephone system from misuse by counterfeiters.
In an effort to provide additional security, some cellular systems and other wireless services, authenticate mobile units based on the radio frequency (RF) transmission of data by the mobile unit during a call set-up process. Rather than identify the mobile unit by its ESN and MIN alone, the system identifies a mobile unit by its transmission characteristics. In this manner, the cellular system operator can reject calls from illegitimate mobile units even when those mobile units transmit valid ESN and MIN numbers. For example, in U.S. Pat. No. 5,005,210 issued to Ferrell on Apr. 2, 1991 ("the Ferrell patent"), a system is described that analyzes certain transmitter characteristics in an effort to identify the transmitter type. The system in the Ferrell patent analyzes the manner in which the modulator makes a transition to the designated carrier frequency. This transient response is used to identify the type of transmitter.
While the Ferrell patent describes one class of transmission characteristics that can be used as a fingerprint, other transmission characteristics are also known in the art. For example, U.S. Pat. No. 5,420,910 issued to Rudokas et al. on May 30, 1995 ("the Rudokas patent"), describes an identifier, such as a radio frequency signature, that can be used to positively identify a valid cellular telephone or a known fraudulent telephone. Other types of signature authentication systems are also known in the art and need not be described herein. These transmission characteristics, from whatever source they are derived, can be processed in different manners to create a "fingerprint" of the individual transmitter. The analogy with fingerprints is used because each fingerprint is believed to be completely unique. The transmitter fingerprint can be used to determine whether the transmission characteristics of an unknown transmitter match the stored fingerprint of the authentic transmitter corresponding to the transmitted ESN and MIN. In such manner, the fingerprint is used with cellular telephone calls to authenticate the cellular telephone.
The fingerprint authentication systems all require at least one transmission characteristic waveform, known to be generated by the authentic cellular telephone, to be used as a reference waveform for the fingerprint authentication system. Some systems may rely on more than one reference waveforms to generate the fingerprint. One drawback of these fingerprint authentication systems is making sure that only authentic reference waveforms are included when delivering the fingerprint to be stored. The fingerprint is corrupted if it includes as reference waveforms transmission characteristics from illegal cellular telephones using the ESN and MIN of the authentic cellular telephone for which the fingerprint is being defined.
Some systems take steps to assure that the reference waveforms are from the authentic cellular telephone. For example, some cellular telephone systems monitor the telephone numbers dialed by the cellular telephone in an effort to detect a pattern of use. The commonly called telephone numbers are sometimes referred to as "valid dialed digits." In these fingerprint authentication systems, reference waveforms may be collected from the cellular telephone when it dials one of these valid dialed digits. This helps assure that it is the authentic cellular telephone generating the reference waveforms.
However, even with the assurance that the cellular telephone is authentic, the reference waveforms themselves are often collected under adverse or varying conditions thus leading to inaccurate fingerprints. For example, the signal strength of the cellular telephone transmission often varies based on the location of the cellular telephone and other factors such as the strength of the battery in the cellular telephone. Thus, the received signal, which is used as the source of the reference waveforms, may be a noisy or otherwise poor quality signal and thus provide a poor quality fingerprint. Another variable factor leading to poor quality fingerprints is the operating frequency or channel of the cellular telephone. As is known in the art, each cell in a cellular telephone network operates on a different frequency. The particular frequency or channel on which the cellular telephone is operating may also affect the quality of the fingerprint because the reference waveforms may be different at different frequencies. Furthermore, environmental factors, such as temperature of the cellular telephone, can alter the reference waveforms and thus adversely affect the fingerprint generated.
In addition to variable conditions related to the cellular telephone itself, external environmental factors may also affect the quality of the reference waveforms used to generate the fingerprint. For example, the cellular telephone transmission may be affected by other nearby transmitters, or other sources of radio frequency interference (RFI), as well as natural phenomenon such as sun spots, which may interfere with or disrupt normal radio communications.
Therefore, it can be appreciated that there is a significant need for a system and method to collect reference waveforms and formulate an authenticating fingerprint which eliminates these problems. The present invention provides this and other advantages as will be illustrated by the following description and accompanying figures.